This invention relates generally to control circuits for operating load devices and, more particularly, to an improved circuit for starting and operating arc lamps.
Although applicable to a variety of load devices, such as arc discharge lamps, the present invention is particularly useful in connection with short arc metal halide lamps in which a pair of electrodes are spaced to provide an arc in the order of one quarter inch within a quartz envelope having a filling including selected metal additives in the form of halogen compounds, generally known as iodides, to achieve desired alterations in lamp discharge characteristics. Such lamps, which are often used for applications such as light sources in projectors, impose severe operating and starting requirements on the associated power supply. Typically, such lamps have a start or breakdown voltage in the order of thousands of volts for pulsing the lamp to initiate electrical conduction across the arc spacing. As the lamps begin to operate, they have a requirement for an intermediate voltage in the order of hundreds of volts for carrying the conduction breakdown region of the lamp through to a point where the lamp can be continuously operated with a comparatively low supply voltage, e.g. about 40 volts.
As mentioned above, applications for which such metal halide lamps are particularly suited are projection lighting systems that may be used in theaters or class rooms, and also in homes. It is desirable, therefore, that the power supply for such a lamp be compact, light weight, and operable from a conventional 120 volt, 60 Hertz source of alternating current line voltage. Further, it should be safe and relatively simple to operate. With respect to performance, the quality and uniformity of color temperature and light output from lamp to lamp and over the life of a given lamp are significant factors in such applications. In order to extend useful lamp life and maintain the desired uniform output characteristics, the lamp supply circuit should maintain a constant power level notwithstanding the typical variations in line voltage.
Various types of ballast circuits are well known in the art for controlling the operation of arc discharge lamps. For example, U.S. Pat. No. 3,987,339 describes a transformer for ballast circuit for providing constant power to a high intensity discharge lamp. Although affording desired power control, this transformer approach presents the disadvantages of adding to the weight, bulk and cost of the power supply. U.S. Pat. No. 3,967,159 employs an over voltage feedback circuit from the load and a duty cycle modulator in a comparatively complex circuit for providing a constant current for a laser or gas discharge lamp. The circuit does not directly sense line voltage variations, and does not discuss maintaining a constant source of power. U.S. Pat. No. 3,700,962 describes a doubler switching ballast circuit for controlling mercury arc lamps. A photoconductive element responds to variations in the light output of the lamp to control doubler switching so as to automatically vary the effective power supplied to the lamp to maintain a more constant light output under conditions of varying supply line voltage or lamp aging. Again, there is no direct sensing of the line voltage and a form of load feedback circuit is required; further, the patent refers to varying power and does not mention maintaining a constant source of power. U.S. Pat. Nos. 3,328,673 and 3,344,311 describe control circuits which do show direct line voltage sensing means employing an incandescent lamp and photoconductor. Constant current is maintained through the load by a pair of AC operated phase controlled switches. In addition to having a somewhat cumbersome arrangement, the last-mentioned circuits employ a form of chopper control with the attendant disadvantage of providing a source of possible radio frequency interference (RFI) problems. The following patents are of general interest with respect to starting and operating circuits for arc lamps and include a reactor ballast on the AC side of the full wave rectifier: U.S. Pat. Nos. 3,467,886; 3,780,342; and 4,045,709.
In contrast to many of the aforementioned prior art circuits, the present invention provides a constant power source for a load, such as an arc lamp, without connecting feedback monitoring circuits to the lamp. Further, the current flow in the resonant circuit has a half sine wave pulse shape, which causes less RFI than the rectangular pulse shape of the prior art switching circuits. In addition to providing a constant power regardless of line voltage variations, the control circuit of the invention also provides the necessary voltages for lamp starting in an efficient, low cost manner which minimizes the bulk and weight of the system.